Pneumatic acoustic source employing electromagnetically controlled valve

ABSTRACT

The specification discloses an acoustic source having a chamber for receiving gas. A valve is provided for confining gas under pressure in the chamber. An electromagnet is employed for holding the valve in its closed position and for releasing the valve to release the pressurized gas from the chamber by way of a port extending through the electromagnet. In the valve&#39;&#39;s closed position, a seal is formed between the two opposing surfaces of the electromagnet and the valve around the port and spaced inwardly from the outer boundaries of the two surfaces. In the preferred embodiment, a cylindrical member is supported to extend from the electromagnet and to surround the valve when it is in its closed position. A clearance exists between the outer periphery of the valve and the inside diameter of the cylinder whereby there is a lack of fluid seal formed between the cylinder and the valve.

United States atent [72] Inventor George B. Lopcr Duncanv'llle, Tex.

[21] Appl. No. 6,092

[22] Filed Jan. 27, 1970 [45] Patented Oct. 19, 1971 [73] Assignee Mobil Oil Corporation Continuation-impart of application Ser. No. 663,800, Aug. 28, 1967, now Patent No. 3,506,085, dated Apr. 14, 1970.

[54] PNEUMATIC ACOUSTIC SOURCE EMPLOYING ELECTROMAGNETICALLY CONTROLLED VALVE 4 Claims, 11 Drawing Figs.

[52] US. Cl 181/5,

[51] Int. Cl G0lv 1/02 [50] Field of Search 181/.5 AG,

[56] References Cited UNITED STATES PATENTS 3,327,264 6/1967 Rodaway 251/129 PRESSURE cnmasn Q ELECTRO MAGNETIC ASSEMBLY 3,121,212 2/1964 Weberetal.

181/.5 A 3,381,931 5/1968 Boonshaft et a1. 251/30 3,322,232 5/1967 Chalmers et a1. 181/.5

ABSTRACT: The specification discloses an acoustic source having a chamber for receiving gas. A valve is provided for confining gas under pressure in the chamber. An electromagnet is employed for holding the valve in its closed position and for releasing the valve to release the pressurized gas from the chamber by way of a port extending through the electromagnet. 1n the valve s closed position, a seal is formed between the two opposing surfaces of the electromagnet and the valve around the port and spaced inwardly from the outer boundaries of the two surfaces. In the preferred embodiment, a cylindrical member is supported to extend from the electromagnet and to surround the valve when it is in its closed position. A clearance exists between the outer periphery of the valve and the inside diameter of the cylinder whereby there is a lack of fluid seal formed between the cylinder and the valve.

PATENTEUUEI 19 I97! I 3,618,824

SHEET 1 or s GEORGE B. LOPER 34a 27a INVENTOR ATTORNEY PATENTEDUET 19 197i SHEET 2 BF 5 I l i i I l l .lal n LR H 9 J Wm N 1 s wllTlKl LN m TS. n J an I I] l ..W E W v A R |||I 6 R5 0 3 4 u E In m mm =w P f G a A W .H v I I I W OW+ w F ,v F 7| 3 QIABIIIL w l i l l llilllllillil PRESSURE CHAMBER 25 ELECTRO- MAGNETIC I ASSEMBLY I DECELERATION CONTAINER PNEUMATIC RETRACT SYSTEM PATENTEDum 19 [an I sum 5 or 5 FIGJO A FIG. 9

PNEUMATIC ACOUSTIC SOURCE EMPLOYING ELECTROMAGNETICALLY CONTROLLED VALVE US. This application is a continuation in part of US. application Ser. No. 663,800, filed Aug. 28, 1967, now US. Pat. No. 3,506,085, issued Apr. 14, 1970.

BACKGROUND OF THE INVENTION This invention relates to a novel arrangement in a pneumatic sound source for increasing the energy output with a minimum of machine tolerances required between moving mechanical parts.

In US. S. Pat. No. 3,506,085 there is disclosed a pneumatic acoustic source having an electromagnet comprising a coil and magnetic structure which is controlled to hold and release the sources gas pressure relief valve. The source comprises a chamber for receiving gas and holding gas under pressure. A chamber release port extends through the electromagnet. The valve is formed of magnetic structure and is supported for movement between a closed position adjacent the electromagnet and an open position for closing and opening the port. The coil is energized to form a magnetic force for holding the valve closed for confining pressurized gas in the chamber. Seal means is provided for forming a fluid seal between the two opposing surfaces of the electromagnet and valve and around the port when the valve is closed. An acoustic pulse is generated by deenergizing the coil to reduce the magnetic force to allow the pressurized gas in the chamber to move the valve to its open position to release the gas through the port.

SUMMARY OF THE INVENTION In accordance with the present invention, skirt means is supported to surround the space formed between the electromagnet and the valve means for a predetermined distance of travel of the valve means as it moves away from the closed position. The skirt means confines a substantial portion of pressurized gas released between the two surfaces during the travel of the valve means over the predetermined distance for rapid release when the valve means reaches the open position. There is, however, a lack of fluid seal between the skirt means and either the valve means or the electromagnet means whereby some pressurized gas is exposed to water in which the source is to be inserted as soon as the seal between the two surfaces is broken but before the valve moves over the predetermined distance.

In the preferred embodiment, a cylindrical means is supported to extend from the electromagnet means for receiving the valve means. The cylindrical means and the valve means have dimensions which result in a lack of fluid seal between the inside surface of the cylindrical means and the outside surface of the valve means. Such an arrangement has advantages in that it results in the production of an increased energy output while eliminating the need for accurate machine tolerances between moving mechanical parts. Also eliminated is the need of sliding surfaces which otherwise will wear.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a pneumatic sound source employed in marine seismic surveying operations;

FIG. 2 illustrates in detail the internal and external structure of the source of FIG. 1;

FIG. 3 illustrates the source of FIGS. 1 and 2 with its valve moving toward an open position;

FIG. 4 illustrates a preferred embodiment of the present invention;

FIGS. 5 and 6 illustrate traces useful in understanding the present invention;

FIGS. 7-9 illustrate another embodiment of the present invention; and

FIGS. 10 and II illustrate two other embodiments of the present invention.

DESCRIPTION OF THE OPERATION OF THE PNEUMATIC SOUND SOURCE Referring now to FIG. l,'an acoustic source 20 is shown supported in water from a boat 21 by a cable 22. As can be seen from FIG. 2, the acoustic source comprises enclosing wall structure 24 forming a pressure chamber 25 and which has a port 26 through which pressurized gas is released from the chamber. A quick-opening valve 27 is provided for opening and closing the port. When the valve is in a closed position, the chamber is pressurized with high gas pressure. Ringshaped seal 28 seals the outlet port. At a desired time, the valve 27 is actuated for sudden downward movement, as illustrated in FIGS. 1 and 3, to allow the high gas pressure to escape rapidly into the water by way of the outlet port 26, thereby generating an acoustic pulse.

The quick-opening valve 27 is made of magnetic material, such as mild steel. It is supported by a retract system 31, including cylinder 32 and piston 33, for movement to a closed position adjacent structure 34 forming the chamber port 26. Structure 34 is constructed of magnetic material, preferably mild steel. Positioned within ring-shaped slot 35, formed in structure 34, and surrounding the chamber port is an electrical coil 36. This coil, along with structure or core 34, forms an electromagnet. In operation of the source, coil 36 normally is energized by current applied thereto from DC source 37, leads 38, normally closed switch 39, and leads 40 of cable 41. This current produces a magnetic field as illustrated at 42. This field in turn produces or develops a magnetic force which is applied to and acts on the valve when it is in its closed position to hold the valve closed against the gas pressure within the chamber 25. When it is desired to actuate or trigger the valve 27 to generate an acoustic pulse, switch 39 is actuated to interrupt the flow of current to the coil. This allows the magnetic field and resulting force to decay to a level insufficient to hold the valve in its closed position against the pressure in the chamber 25. At this point the high gas pressure in the chamber then moves the valve 27 rapidly downward to its open position for release into the water to generate an acoustic pulse. Water in container 43 slows the valve 27 at the end of its opening movement. Following the generation of an acoustic pulse, retract mechanism 31 moves the valve 27 to its closed position for a repeat cycle.

DETAILED DESCRIPTION OF THE INVENTION Secured around the core 34 and extending therefrom is a skirt or cylinder 50. In one embodiment, it may extend from the core 34 for a distance of about 1 inch. The dimensions of the valve 27 and the cylinder 50 are such that there is a lack of fluid seal between these two members. Thus, machine tolerances between the valve 27 and the cylinder 50 are maintained at a minimum and the need of sliding surfaces is avoided. However, the use of the cylinder results in an increase in energy output. In this respect, after the valve is released and has moved away from the seal 28, pressurized gas from the chamber 25 flows through port 26 and between the two opposing surfaces 27a and 340 (FIG. 3) of the valve 27 and core 34, respectively. The small clearance between the valve 27 and the cylinder 50 and the water within this clearance impedes the flow of gas through the clearance. Thus, substantially the full force of the pressurized gas is in effect confined between the two opposing surfaces of the valve 27 and the core 34 and within the extending skirt 50 until the valve reaches the end of the skirt. Confining substantially the full force of the gas and allowing it to act over the entire surface 27a of the valve 27 results in the valve 27 being accelerated to a high speed before it reaches the end of the skirt 50. This results in a very rapid release of pressurized gas into the water, thereby enhancing the acoustic energy generated.

Referring to FIG. 4, a preferred embodiment of the source 20 is illustrated. In this embodiment, a cylinder 60 extends from the core 34 and comprises a skirt portion 60a. Windows or vents 60b are formed through the cylinder 60 at the end of the skirt portion 60a. The other end portion 600 of the cylinder 60 forms a deceleration container for slowing movement of the valve 27 Tests were conducted with the source of FIG. 4 with and without the skirt 60a to determine the effect of the skirt on the energy output. The length of the skirt 60a from the core 34 to the vents 60b was of the order of 1 inch. Its effect was removed by enlarging the vents 60b whereby they were extended to the surface 340 of the core facing the valve 27. The source tested had a valve 27 whose outside diameter was 14.040 inches. The inside diameter of the skirt portion 601: was 14.080 inches. The total cross-sectional area of the clearance between the valve and the skirt portion 60a was of the order of 0.9 square inch. The chamber port 26 of this skirt was 5 inches in diameter. The chamber was pressurized to 500 p.s.i. above atmospheric pressure and located in water to a depth of 40 feet. FIG. 5 shows the output of the skirted source, while FIG. 6 shows the output of the unskirted source. These traces illustrate the pressure detected in the water as a function of time. Pressure increases are shown in the downward direction. The rimary pressure pulse detected and analyzed during each test is the crosshatched portion of each trace. Within the seismic energy band of 13.7-50.8 hertz, the energy produced in joules as indicated by the primary pressure pulse of the trace of FIG. 5 was calculated to be 2,360 joules. The energy in joules as indicated by the primary pressure pulse of the trace of FIG. 6 was calculated to be 891 joules. Thus, it can be seen that even though there is a fluid flow clearance between the valve 27 and the skirt 60a, of the source of FIG. 4, the energy produced with the skirted source was much greater than that produced with the unskirted source.

In the embodiments of FIGS. ll-4, the skirt was securely attached to the core of the electromagnet. Referring now to FIGS. 79, there is disclosed an arrangement where the skirt is movably secured to the core 34. The source employed is of the type shown in FIG. 2. The movable skirt 67 comprises a cylinder having a shoulder 68 and a plurality of elongated apertures 69 extending through the sidewall 70 of the cylinder. The skirt 67 is coupled to the core 34 by machine screws 71 which extend through the apertures 69 and are threaded into the core 34. The apertures are large enough to allow the skirt 67 to reciprocate in the direction of movement of the valve 27. In the closed position of the valve, the shoulder 68 fits into an annular slot 72 formed around the outer edge of the valve. As can be seen in FIG. 7, the shoulder 68 has a beveled edge 73. Wire springs 74 normally urge the skirt 67 downward. One end of each spring 74 fits into a groove 75 formed in the top edge of the sidewall 70 of the skirt. The other end of each spring is welded to a sleeve 76 which is secured to the core 34 by a machine screw 77 threaded into the core. The dimensions of the core 34 and the skirt 67 are such that there is a lack of fluid seal between the outside surface of the core 34 and the inside surface of the skirt 67. The operation of the skirt 67 is as follows. As the valve 27 moves away from the core 34, the pressurized air released from the port 26 tries to escape past the skirt 67. The pressurized air strikes the beveled edge 73 and is deflected upward, and a small flow escapes through the slots 69 and between the skirt 67 and the outer surface of the core 34. The skirt is pressed by air pressure against the valve and stays there as both move downward for a certain distance. The machine screws 71 then stop the movement of the skirt 67 and the valve breaks away to provide a sudden opening for full pressure release after the valve has accelerated to high velocity. The arrangement of FIGS. 7-9 also has advantages in that machine tolerances and the need of sliding surfaces are minimized while at the same time the skirt 67 retains a substantial portion of the pressurized gas between the two opposing surfaces of the valve and the core until the valve has accelerated, thereby increasing the energy output.

Referring now to FIG. 10, another embodiment of a reciprocating skirt is illustrated. This embodiment is similar to that of FIGS. 7-9 except that the shoulder 68' is beveled to a point whereby it is triangular in cross section. The edge of the core 34 also is beveled to form a slot with the valve when the valve is in its closed position. The shoulder 68 fits into this slot. Surfaces 73' and 79 deflect the airflow upward between the skirt and the core so as to hold the skirt against the valve during a portion of its downward travel. This system otherwise works the same as that of FIGS. 79.

Referring to FIG. 11, there is disclosed a further embodiment whereby a sleeve or skirt 80 is coupled to the valve 27 and fits into a slot 81 formed in the core 34 when the valve is in its closed position. The dimensions of the skirt 80 and the slot 81 are such that there is a slight clearance between these two members, thereby minimizing machine tolerances and sliding surfaces but resulting in an increase in energy output since the skirt 80 will act to confine a substantial portion of the pressurized gas until the skirt moves away from the core 34.

Referring now to FIG. 2, more details of the source and operating system disclosed therein will be described. The chamber-pressurizing system, power supplies, and control instrumentation are located on the towboat and are illustrated in the dashed box 90. The chamber-pressurizing system may comprise an air compressor 91 coupled to the chamber 25 by way of conduit 92, solenoid-controlled valve 93, and conduit 94, including a check valve 95. The air compressor 91 also is coupled to the retract chamber 32 by way of conduit 92, conduit 96, three-way valve 97, and conduit 98. Valve 93 normally is closed, while valve 97 is a solenoid-controlled valve which normally vents retract cylinder 32 to the atmosphere by way of conduit 98 and vent 99. A timer I00 controls both valves 93 and 97 and, in addition, switch 39, the latter of which normally is closed.

The sequence of operation during one cycle now will be described. In the beginning of this cycle, it is assumed that an acoustic pulse has been generated and the valve 27 has been moved to its closed position. At this time, both valve 93 and valve 97 are in their normal states whereby the flow of air to chamber 25 and to cylinder 32 is blocked and, in addition, cylinder 32 is vented to the atmosphere. Switch 39 normally is closed whereby DC current is applied to the coil 36 to produce the magnetic holding force for holding the valve 27 closed. Timer first causes valve 93 to open to allow air to flow into chamber 25. After chamber 25 has been pressurized to the desired value, timer 100 allows the valve 93 to close to block the flow of air into the chamber 25. Next, timer 100 causes switch 39 to open to interrupt the flow of current to the coil 36 and thereby deenergize the coil. This allows the magnetic field to decay whereby the pressure in the chamber will move the valve 27 to its open position for release of the air into the water. After the valve 27 has opened and the gas pressure in the chamber 25 released, timer 100 allows switch 39 to close to create again the magnetic holding force. At about the same time, timer I00 controls valve 97 to block vent 99 and allow air to flow into retract cylinder 32 by way of conduits 96 and 98. The pressurized air in cylinder 32 acts on piston 33 to move the piston and hence the valve 27 upward to its closed position where the magnetic holding force holds it closed. Timer 100 then causes the valve 97 to block the flow of air through conduit 96 and to vent cylinder 32 to the atmosphere by way of conduit 98 and vent 99.

In one embodiment, the coil 36 may be secured within slot 35 by an encapsulating epoxy. Core 34 is secured to chamber structure 24 by way of rim welded to chamber structure 24 and a plurality of bolts Ill threaded into rim I10 and into core 34. A seal (not shown) may be provided between rim I10 and core 34 to assure adequate sealing.

Deceleration container 43 is a cylindrical member having an interior diameter slightly larger than the exterior diameter of valve 27. The cylindrical walls of the container 43 are welded to circular bottom plate 112 which, in turn, is supported by a plurality of spaced rods 113, coupled to rim I10 and to plate 112 by way of bolts illustrated at 114. As the valve 27 moves downward, the water within the container is squeezed upward between the outside surface of the valve 27 and the inside surface of the container to decelerate the valve.

The cylindrical wall structure of retract cylinder 32 is welded to the bottom surface of plate 112 and to the top surface of plate 115. Plate 1 is supported further by a plurality of spaced rods 116 coupled to plate 112 and to plate 115 by way of bolts illustrated at 117.

Valve 27 is coupled to retract piston 32 by way of coupling rod 120. The lower end of rod 120 is welded to piston 33, while its upper end is secured to valve 27 with a flexible coupling arrangement to provide a cushioning effect and to avoid the necessity of high precision of parts.

The flexible coupling arrangement comprises a circular plate 121 threaded to the upper end of rod 120 and which also is coupled to valve structure 27 by way of bolts 122. The apertures in plate 121, through which the bolts 122 extend, are larger than the stems thereof whereby the plate 121 can move relative to bolts 122. A resilient member 123 is inserted between the plate 121 and the valve 27. The upper end of rod 120 pivots in a rounded aperture formed in the lower surface of the valve 27. Cup-shaped member 124 is threaded to the upper end of rod 120 and is provided to prevent excessive angular movement between the rod 120 and the valve 27 A suitable timer 100 and switching system 39 are disclosed in the aforementioned U. S. Pat. No. 3,506,085.

Referring now to FIG. 4, more details of the source illustrated therein will be described. A mechanical retract spring 130 is employed for moving the valve to its closed position following the generation of an acoustic pulse. This spring has one end supported in cylinder 131 which is coupled to the back end of the valve 27 by machine screws 132. Cylinder 131 is supported for movement by cylinder 133 and bearing 134. Cylinder 133 is supported by an arrangement including an annular plate 135 welded to cylinder end 60c. The end of the cylinder 60 which surrounds the core 34 is coupled to the core 34 by way of machine screws 136. The other end of the spring 130 is supported in an annular-shaped member formed by cylinder 137 coupled to cylinder 138 by spokes 139. Cylinder 138, in turn, is threaded into cylinder 133. Secured to the exterior of cylinder 133 is an annular plate member 140. A truncated, cone-shaped shell 141 is coupled to annular plate 135 and to annular member 140 for additional support for the back end portion of the source. A plate 142 is coupled to annular member 140 by way of machine screws 143. Coupled to plate 142 are fins 146, 147, and 148.

The valve 27 is slowed and stopped at the end of its opening movement by water located in a deceleration container formed by plate 135 and the cylindrical portion 600. As the valve moves within cylinder 60, water within the deceleration container is squeezed between the outside surface of the valve and the inside surface of cylindrical portion 60c to decelerate the valve. Water within the cylinder 131 may flow by apertures 150 extending through its sidewall and by way of aperture 151 extending through plate 142.

In one embodiment, the seal 28 employed in the source may be of the type mentioned in US. Pat. No. 3,506,085. In the embodiment of FIG. 4, it is held by member 160 which is secured to core 34 by machine screw 161. Seal 28 forms a seal with metal plate 162 secured to the valve 27 by way of member 163 and machine screw 164. Wire member 165 coupled to screw 164 and to rod 166 prevents screw 164 from turning.

The supporting cable 22 is coupled to a harness 170 which, in turn, is coupled to nose member 171 and to the vertical tail fin 146. Also coupled to the harness is a strain cable 173 which extends-to the boat 21. The air conduit 94, electrical leads 40, and cable 173 are bound together by tape (not shown) to form a flexible and lightweight conduit which extends to the boat.

At the source, the air conduit 94 extends through tubing 174 to the pressure chamber 25. The electrical leads 40 extend through a single tube 175 coupled to the harness and then through smaller tubes 176 and 177 coupled to the harness and to the source. The electrical leads 40 then extend to a terminal box 178 where they are connected to the two ends of the electrical coil 36.

The source of FIG. 4 may employ the same control system shown at in FIG. 2 except that conduits 96, 99, and 98 and three-way valve 97 are not needed since the source of FIG. 4 employs a mechanical spring for retract purposes.

What is claimed is:

1. A source to be inserted in water for generating acoustic energy in water for application to the earth for exploratory purposes comprising:

a chainber for receiving gas and holding gas under pressure to be released for the generation of an acoustic pulse,

said chamber being formed of rigid wall structure and having a port for the release of pressurized gas,

valve means comprising a member formed of magnetic material supported for movement between a closed position and an open position for closing and opening said port, respectively,

electromagnet means comprising a coil and a member formed of magnetic material for forming a magnetic flux for holding said valve means in its closed position for confining pressurized gas in said chamber and for releasing said valve means for movement to its open position for releasing pressurized gas from said chamber to generate an acoustic pulse,

said electromagnet means having a surface facing in the direction of said valve means,

said valve means having a surface facing in the direction of said electromagnet means,

said port extending through said electromagnet and being smaller in cross section than the cross section of said valve means,

said valve means being supported to move adjacent said surface of said electromagnet when said valve means is moved to said closed position,

seal means for forming a fluid seal between said two surfaces around said port when said valve is in said closed position to seal said port, and

skirt means supported to surround the space formed between said electromagnet means and said valve means for a predetermined distance of travel of said valve means as it moves away from said closed position, said skirt means confining a substantial portion of pressurized gas released between said two surfaces while said valve means travels over said predetermined distance for rapid release when said valve means reaches said open position, there being a lack of fluid seal between said skirt means and one of said members whereby some pressurized gas is exposed to water in which said source is to be immersed as soon as said seal between said two surfaces is broken but before said valve means moves over said predetermined distance. 2. The source of claim 1 comprising: means for coupling said skirt means to said electromagnet means and allowing said skirt means to follow said valve means over said predetermined distance of travel of said valve means, and 7 means for causing said skirt means to follow said valve means over said predetermined distance of travel of said valve means when said valve means moves toward said open position.

3. The source of claim 2 wherein:

said skirt means is coupled to said valve means for movement therewith and extends around said surface of said valve means in a cup-shaped manner for telescoping around the end of said electromagnet means.

4. A source for generating acoustic energy for application to the earth for exploratory purposes comprising:

a chamber for receiving gas and holding gas under pressure to be released for the generation of an acoustic pulse,

said chamber being formed of rigid wall structure and having a port for the release of pressurized gas,

valve means comprising magnetic structure supported for movement between a closed position and an open position for closing and opening said port, respectively,

electromagnet means comprising a coil and magnetic structure for forming a magnetic flux for holding said valve means in its closed position for confining pressurized gas in said chamber and for releasing said valve means for movement to its open position for releasing pressurized gas from said chamber to generate an acoustic pulse,

said electromagnet means being annular in shape and having a surface facing in the direction of said valve means,

said valve means having a surface facing in the direction of said electromagnet means,

said port extending through said electromagnet and being smaller in cross section than the cross section of said valve means,

said valve means being supported to move adjacent said surface of said electromagnet when said valve means is moved to said closed position,

seal meansfor forming a fluid seal between said two surfaces around said port when said valve is in said closed position to seal said port,

said seal means being located to form said seal at a position spaced inwardly from the outer boundaries of said two surfaces, and

cylindrical means supported to extend from said electromagnet means for receiving said valve means,

said cylindrical means and said valve means having dimensions which result in a lack of fluid seal between the inside surface of said cylindrical means and the outside surface of said valve means 

1. A source to be inserted in water for generating acoustic energy in water for application to the earth for exploratory purposes comprising: a chamber for receiving gas and holding gas under pressure to be released for the generation of an acoustic pulse, said chamber being formed of rigid wall structure and having a port for the release of pressurized gas, valve means comprising a member formed of magnetic material supported for movement between a closed position and an open position for closing and opening said port, respectively, electromagnet means comprising a coil and a member formed of magnetic material for forming a magnetic flux for holding said valve means in its closed position for confining pressurized gas in said chamber and for releasing said valve means for movement to its open position for releasing pressurized gas from said chamber to generate an acoustic pulse, said electromagnet means having a surface facing in the direction of said valve means, said valve means having a surface facing in the direction of said electromaGnet means, said port extending through said electromagnet and being smaller in cross section than the cross section of said valve means, said valve means being supported to move adjacent said surface of said electromagnet when said valve means is moved to said closed position, seal means for forming a fluid seal between said two surfaces around said port when said valve is in said closed position to seal said port, and skirt means supported to surround the space formed between said electromagnet means and said valve means for a predetermined distance of travel of said valve means as it moves away from said closed position, said skirt means confining a substantial portion of pressurized gas released between said two surfaces while said valve means travels over said predetermined distance for rapid release when said valve means reaches said open position, there being a lack of fluid seal between said skirt means and one of said members whereby some pressurized gas is exposed to water in which said source is to be immersed as soon as said seal between said two surfaces is broken but before said valve means moves over said predetermined distance.
 2. The source of claim 1 comprising: means for coupling said skirt means to said electromagnet means and allowing said skirt means to follow said valve means over said predetermined distance of travel of said valve means, and means for causing said skirt means to follow said valve means over said predetermined distance of travel of said valve means when said valve means moves toward said open position.
 3. The source of claim 2 wherein: said skirt means is coupled to said valve means for movement therewith and extends around said surface of said valve means in a cup-shaped manner for telescoping around the end of said electromagnet means.
 4. A source for generating acoustic energy for application to the earth for exploratory purposes comprising: a chamber for receiving gas and holding gas under pressure to be released for the generation of an acoustic pulse, said chamber being formed of rigid wall structure and having a port for the release of pressurized gas, valve means comprising magnetic structure supported for movement between a closed position and an open position for closing and opening said port, respectively, electromagnet means comprising a coil and magnetic structure for forming a magnetic flux for holding said valve means in its closed position for confining pressurized gas in said chamber and for releasing said valve means for movement to its open position for releasing pressurized gas from said chamber to generate an acoustic pulse, said electromagnet means being annular in shape and having a surface facing in the direction of said valve means, said valve means having a surface facing in the direction of said electromagnet means, said port extending through said electromagnet and being smaller in cross section than the cross section of said valve means, said valve means being supported to move adjacent said surface of said electromagnet when said valve means is moved to said closed position, seal means for forming a fluid seal between said two surfaces around said port when said valve is in said closed position to seal said port, said seal means being located to form said seal at a position spaced inwardly from the outer boundaries of said two surfaces, and cylindrical means supported to extend from said electromagnet means for receiving said valve means, said cylindrical means and said valve means having dimensions which result in a lack of fluid seal between the inside surface of said cylindrical means and the outside surface of said valve means. 